1. Field of the Invention
The present invention relates to a power converter, and particularly to a power converter with a loop-compensated filter.
2. Description of the Prior Art
Power converters are generally used in modern power electronics industry to convert electric power from one form to another. They are mainly used in power supplies, motor control systems, high-voltage dc transmission, and other power conversion applications. Depicted in FIG. 1 is a schematic circuit diagram illustrating a conventional power converter 10 used to convert an ac source 11 to a regulated or controllable output power, which provides current or voltage across a load 12, and may be ac or dc. In this circuit, the source 11 is firstly filtered by a full-wave bridge rectifier 14 comprising diodes D1 to D4 configured as shown in the diagram. Next, a capacitor 16 is used to smooth the rectified voltage from the rectifier 14, so that the ripple component in the rectified voltage could be largely reduced. The smoothed voltage from the capacitor 16 is further controlled by a controller 18, which controls switches S1 to S4 in order to generate the ac or dc output power. One of the disadvantages for this conventional power converter 10 is the need of the bulky and costly electrolytic capacitor 16, especially in high-power converters.
One solution to this problem is the use of pulse width modulation (PWM) to achieve voltage regulation with a capacitor 16 of reasonable size. FIG. 2A shows a circuit diagram of a conventional PWM converter 20. A control signal Vc is generated from an error amplifier 21, which amplifies an error signal between an actual output Vo and a reference Vref. The control signal Vc then controls the duty ratio of the switch (such as the switch S1, S2, S3, or S4 in FIG. 1) by comparing the control voltage Vc with a sawtooth waveform 23 through a comparator 22. This control of the switch duty ratio adjusts the voltage across the inductor L and hence the inductor current i.sub.L (which feeds the output voltage), and eventually brings the output voltage Vo to its reference value Vref. Disadvantages of this circuit 20 are poor dynamic response, and slow response to sudden input changes. Another PWM converter 24 in the art is shown in FIG. 2B, wherein the control voltage Vc directly controls the output inductor current i.sub.L in a current-mode control by using an additional inner control loop, which compares inductor current i.sub.L with the control voltage Vc. Although this PWM circuit 24 improves the problems occurred in the PWM circuit 20 of FIG. 2A, however, unstability and complexity are incurred in this circuit 24.
Nowadays, in order to improve switching efficiency, a variable-frequency resonant converter configuration is disclosed. Illustrated in FIG. 3 is a schematic block diagram of a conventional variable-frequency resonant converter 30, which mainly includes a comparator 32, a voltage-controlled oscillator (VCO) 34 and a drive circuit 36. The use of inductor-capacitor LC resonating elements thus improves switching efficiency. However, the disadvantages of this variable-frequency resonant converter 30 are its high cost and complexity.
Owing to the inherent drawbacks, such as limited frequency band and response, in both PWM converters and resonant converters, the ripple component in the converted output could not be adequately reduced. Accordingly, there is a need to provide a scheme to substantially reduce this ripple component.